The present invention relates to telescopic booms used for lifting loads.
Telescopic booms have been used on cranes, for lifting various loads, and have also been used on aerial lift platform apparatus, for supporting a workman's platform at the outer end of the boom. These booms are made in desired lengths, and some have extended as high as approximately 110 feet, more or less. In the construction of booms of such size, a number of problems arise. When one section of the telescopic boom is caused to extend outwardly of the larger section which surrounds it, and is then caused to retract, it is necessary that all of the elements which are intended to pass adjacent to each other are prevented from striking in such a manner as to lock the boom against further retraction. This problem has been recognized to a certain extent in the prior art. For example, Grove U.S. Pat. No. 3,243,052 provides a metal skid with a rounded end on the outer end of a cylinder box, in a multi-section telescopic lifting boom. The skid extends downwardly from the cylinder block, and is intended for permitting the bottom plate of a section of the boom to move past the skid, without locking. Brown Et Al U.S. Pat. No. 3,620,579 provides a plurality of cylinders with inclined elements or ramps for this same purpose. The constructions heretofore provided, however, have not taken into consideration the possibility that other elements than those at the bottom of the various boom sections may have meeting surfaces which could cause a lockup of the boom on retraction, nor have there been provided other than simple inclined metalic surfaces for this purpose.
Further, it is known to extend and retract the sections of a telescopic boom by use of both a hydraulic motor and chains or cables. For example, Grove U.S. Pat. No. 3,809,249 discloses a telescopic boom having three sections, and a cylinder box. A hydraulic motor or cylinder has an end pin-connected to the base section, and has the outer end of the piston rod pivotally connected to a cylinder box, which extends rearwardly from the pin connection, and is, itself, connected at its rear end to the rear end of the mid section. Sprockets are carried near the rear end of the mid section and at the front end of the mid section, and a chain extends from the front of the base section rearwardly around the rear sprocket, and is engaged with the rear of the fly section, the chain, or a companion chain, extending from the rear end of the fly section forwardly around a sprocket at the forward end of the mid section, and thence rearwardly to an anchor at the outer end of the base section. By this construction, the cylinder causes the mid section to extend and retract, and movement of the mid section causes the fly section, also, to extend and retract, through the chains just described. Where booms are intended to be of greater length than may be suitably constructed in three sections, they are made of four, or more, sections, and in view of the additional sections, additional problems are encountered. For example, where the cylinder box is pivoted, it is possible that it will engage the chain or chains for the outer section or sections of the boom, and such engagement is required to be prevented, to avoid lockup, interference or undue wear on the parts of the boom. It has been proposed to provide a construction in which an elongated spacer is provided for supporting a cylinder box, and a wear pad is positioned between a pair of cables for supporting the mid section of the boom on the base section. Such prior art disclosures are deficient, however, in providing a relatively friction-free construction for longitudinal movement of the cylinder box relative to a section of the boom, together with maintaining the elevation of the cylinder box so as to prevent interference with the actuating chains.
In connection with the provision of chains for actuating an outer section of a boom, such as is shown in Grove U.S. Pat. No. 3,809,249, there have been provided anchors for the chains which may be tightened, so as to thereby tighten the chains, as they wear under use and tend to lengthen. Such anchors have been provided individually for each chain, and are of a threaded nature. It is difficult in such constructions, however, to tension the chains equally.